C4 Upgrade Air Management to Improved Fuel Delivery - Got Air?

Here are the induction system parts we'll be replacing in this job. Shown are thenew intake manifold and runners, throttle body (and box), mass air flow (MAF) sensor,EGR insulation, and manifold gaskets that we had to purchase separately asthey were, surprisingly, not included with the manifold.

LAST time, our "Got Fuel?"(August 2007) story detailed the proceduresfor upgrading fuel injectors andthe fuel-pressure regulator. This month,we will explain air-managementupgrades to take the best advantage ofthe improved fuel delivery.

More fuel is usually better. But only ifmore air is available to make most efficientuse of the enhanced fuel/air mixture.After all, if properly managed,more air/fuel mixture translates intomore ponies to the pavement. Havingupgraded our fuel injectors and fuelpressureregulator, we decided now wasthe time to install a high-performancethrottle body, runners, and intake manifoldin our project '87.

Once again we turned to the folks atMid America Motorworks to see whichpieces they recommended to complementthe modifications made previously. Theyrecommended Accel's trio of throttlebody, intake runners, and intake manifold,so we ordered up these parts andwent back to re-read the previous storyon fuel-system upgrades because all ofthose pieces would have to come backoff for this job. Certainly it makes senseto do both jobs at the same time if yourcheckbook can withstand the shock. Butif not, we think doing the fuel upgradesfirst is both more affordable and lessintimidating, so when you're ready totackle the manifold and runner job, you'llbe more confident.

We'll say right up front that this jobdid not go as well as previous jobs for anumber of reasons. Access to bolts andparts was, in some cases, difficult. Andwe learned the hard way that manypieces had to come off and go back on ina very specific order, so trial fitting duringreassembly is critical. Further, althoughthe instructions with the manifold advisethat some modifications might be necessaryfor everything to fit right, we weresurprised and disappointed at the sometimesobvious interference points that wefelt should have been engineered betterbefore the final design was released.

And, finally, there were areas like theport size in the new runners that weremuch larger than the ports in the originalplenum, leaving what we felt was a tooabruptchange in diameter. We addressedthat matter by milling the ports in theplenum to match those of the new runners.And the passage for the cold startenrichment injector (the so-called ninthinjector) was simply machined in thewrong place and in the wrong shape. Asa result, we ended up spending time onour machinist's Bridgeport making modificationsthat shouldn't have been necessary.Finally, we found that the new intakemanifold did not come with manifold-tocylinderhead gaskets, so they had to bepurchased separately.

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C4 Upgrade Air Management to Improved Fuel Delivery - Got Air?

Here are the induction system parts we'll be replacing in this job. Shown are thenew intake manifold and runners, throttle body (and box), mass air flow (MAF) sensor,EGR insulation, and manifold gaskets that we had to purchase separately asthey were, surprisingly, not included with the manifold.

01 We removed the metal PCV tube from the throttle body at the front and thevalve cover vent at the rear.

02 In removing the insulated EGR tube, we observed that the insulated wrappingwas coming apart, so we'll be installing new wrapping during reassembly. While theexisting insulation was adequate, we're replacing it primarily for cosmetic reasonswith a nice piece from Mid America Motorworks.

03 After the MAF sensor is off, removingfour bolts allowed us to lift off thethrottle body. Note the plugs that wehad installed when we previouslybypassed the passage of coolantthrough the throttle body. While thiscoolant flow pre-heats the air for bettercold engine performance, it's not anasset during quarter-mile runs whenthe engine is fully warmed up.

04 The plenum can be lifted off afterthe runner-to-plenum bolts have beenremoved. You can lift off the plenumwith one runner still in place, which willmake it easier to remove the oneremaining runner bolt on either sidethat points from the centerline of theengine out toward the side.

05 With all the intake manifold boltsremoved, you can now lift off the manifold.It may be necessary to pryslightly to break loose the gasket surfaces.Be very gentle, and choose yourlocation carefully to avoid damaginganything.

06 It is necessary to transfer thisshield from the original intake manifoldto the new Accel manifold. This splashshield prevents build-up of oil residueon the underside of the manifold.

07 We chose to use thread locking compoundon these shield bolts in order toprevent the catastrophic results of oneof these bolts dropping into internalareas of the engine.

08 We installed the new manifold, beingcareful not to dislodge the new gaskets.Accel's instructions are very specificin directing you to use O2-friendlyRTV sealer and not preformed manifoldend gaskets. They also specify usingthis RTV on both sides of the intakemanifold side gaskets as well, which iscontrary to what most gasket manufacturersrecommend. We took the instructionof the gasket makers on this one.

09 It's important that intake manifoldbolts be torqued to the proper value, inthe proper sequence.

10 Note the difference in the diametersof the ports between the new runners onthe left, and the original runners on theright. The difference is about 0.100-inch--a significant difference that results inenhanced breathing through the inductionsystem. The difference is apparent withthe new gasket positioned over the portsof the original runner.

11 We found that Accel's torque specs and sequence were very different fromthose recommended by GM. The factory manual calls for 25-45 lb-ft per their diagram.We chose to use the 25 lb-ft and the sequence specified by Accel for theirmanifold. This sequence seems more intuitive since it starts at the center and spiralsoutward, which is the pattern we've seen most often for intake manifolds andcylinder heads.

12 With the new runner shown at thebottom and the new gasket placed overthe ports of the plenum, it's clear thatthe ports of the plenum are not wellmatchedto the runners. Our friends atHiway Cycle & Machine fixed this in shortorder, enlarging the port diameter about0.150-inch.

13 We were very disappointedin the size, shape, andlocation of the port in theleft-side runner for the coldstart enrichment injector(the so-called 9th injector).It is clearly obstructed bythe gasket and did not alignwith the mating port on theplenum. We had it carefullymachined to the proper diameterwith a 45-degree bevel toensure smooth air flow.

14 During disassembly we found thattime and temperature had taken theirtoll on several electrical connectors, sothis was the obvious time to replacethem. This is the idle air control (IAC)connector--the broken tab is apparenton the original connector on the left,the new connector on the right.

15 Likewise, the broken throttle positionsensor (TPS) connector is shown onthe left, and the new one on the right.General Motors thoughtfully made theseconnectors in different configurationsso they can't be interchanged duringreassembly.

16 We lowered the modified plenum intoplace, with the left (driver) side runneralready bolted securely in place. Sincethe one inboard bolt on this runner isdifficult to access, we tightened up thisrunner before installing the plenum. Thenwe installed the other runner, for whichthe inboard bolt is a bit easier to reach.

17 We had to remove the EGR controlsolenoid in order to gain access to thepassenger's side inboard runner bolt. Herewe're re-installing the solenoid to itshome under a thermostat housing bolt.

18 This is the right-side inboard runnerbolt.

19 This is the reason we bolted up theleft-hand intake runner before installingthe plenum. This is the inboard bolt at therear inside of the runner and, as you cansee, it is difficult to access with theplenum in place. The new Accel runners aresupplied with Allen head bolts replacingthe original Torx bolts.

20 Here are two of the fourbolts that hold the fuel rail tothe intake manifold. Duringreassembly, it is importantthat all eight of the injectorsare properly seated beforetightening these four bolts.

21 We reinstalled and tightened the twofuel lines using new O-rings to ensure aleak-free assembly. We also took theopportunity to replace the water pumpbypass hose while it was accessible.

22 During reassembly, we found the vacuumtube for the PVC valve interfered withthe reinstallation of the left-side runner,so we had to remove the runner and positionthis vacuum tube before tightening theinjector rail and installing the runner. Alsonote the tight clearance at the 9th injectorin the lower lefthand corner of thisphoto. This is the part we were referringto when describing the issue of specificorder that we had to learn by equal partsof trial and error.

23 The fuel supply line for the 9thinjector is sealed with an O-ring thatmust be positioned carefully to avoiddamage during tightening.

24 This coolant fitting must be swappedfrom the old intake manifold to the new one.

25 After having our friendly localmachinist open up the ports in the plenumto match those in our new, larger runners,there was still further machiningnecessary. We had him relieve the innersurfaces of the various bolt bosses inorder to eliminate all possible restrictionsto intake air flow.

26 We also machined out the bridgesin the front of the plenum in theinterest of reducing restrictions atthat location in the air-intake system.You can see that the bridge inthe left port has been machined away,with the right side not yet done.While we believe this will enhanceperformance, we think removal ofthese bridges may have resulted in aslight induction system whistle duringcold engine operation.

27 Here you can see how much the portsin the plenum had to be enlarged to matchthose in the new runners.

28 Shown here is the vacuum fitting forthe power brake booster at the left rear ofthe plenum. We found a slight interferencebetween this fitting and the new runner(shown in the next photo) and had to grinda relief in the runner to clear this fittingand the vacuum line that connects to it.

29 Here you can see how close a fitthat vacuum line is to the runner; youcan also see the radius we had togrind out of the new runner. A smoothradius is essential, since any sharpedge or corner can create a stressriser that can serve as a startingpoint for a stress crack.

30 The scribe mark shows where wehad to grind to get the small runnerport to match the corresponding porton the plenum to feed the ninth injector.If you look closely, you'll alsonote casting marks and flash in theport to the right. We carefully groundout these irregularities from eachport in order to minimize restrictionsto smooth air flow.

31 Note the very tight fit of the ninthinjector to the valve cover, resulting fromthe configuration of the new runner intowhich the injector fits. We chose to grind aslight radius in the valve cover to ensureclearance.

32 We found these snap-on, ball-type hexand torx drivers to be especially helpful inreaching through odd angles in order toloosen the original Torx runner bolts andreplacement indented hex bolts.

33 Among the modifications we hadto make was slightly crushing thecrankcase fresh air tube in order toclear the front intake runner andbolt boss on the passenger's side.

34 We had to perform similar "caressing"of the fresh air tube to clear the rearvalve cover bolt as well.

35 This shot of the left front of theengine shows the bracket that angles upand forward to support the A.I.R. pump. Wefound we had to grind the corner of the bosson the manifold for clearance for thisbracket, and also had to add several washersunder the bolt head because the tappedhole was too shallow to allow us to re-usethe original bolt.

36 In preparing the new throttlebody for installation, we first had totransfer the throttle position sensor(TPS), shown at the top, and the idleair control (IAC), shown at the bottom,to the new throttle body.

37 The A.I.R. pump bracket is also visiblein this photo, which shows installation ofthe new throttle body.

The 2013 Mid America Motorworks' Funfest marks plenty of anniversaries and celebrations for the Corvette, Mid America Motorworks, and the Funfest event! Literally thousands upon thousands of cars and spectators converged in Effingham, Illinois, for a celebration of all things Corvette. » Read More